The present invention relates to the acquisition of electromagnetic signals. It relates more particularly to the acquisition of such signals received from a body part, in particular a human or animal body part, in response to an external electromagnetic solicitation.
Various methods for acquiring signals are known, in particular in the magnetic resonance imaging (MRI) field. These methods have common characteristics.
They generally consist in subjecting the body in question to a high-intensity magnetic induction B0, typically between 0.1 and 3 Tesla. The effect of this induction is to orient the magnetic moments of the protons of the hydrogen contained in the water molecules of the body in a direction close to the main direction of the magnetic induction B0.
The body part imaged is then subjected to a radiofrequency wave applied perpendicular to the magnetic induction B0 and the frequency of which is typically adjusted to the Larmor precession frequency of the hydrogen nucleus in the magnetic induction B0 in question. This frequency is proportional to the intensity of the magnetic induction B0 and has the specificity of bringing into resonance the protons of the hydrogen contained in the water molecules of the body. By way of example, for an induction B0 of 1 Tesla, the corresponding Larmor frequency is in the region of 42 MHz.
Immediately after the transmission of this radio frequency wave, the magnetic moments that have been subjected to the wave begin to oscillate around their equilibrium position and again take up a position along their original direction, close to that of the magnetic induction B0. This phenomenon is known as proton relaxation.
During the relaxation, each water proton that has come into resonance creates, as a result, a relatively weak electromagnetic signal, called a magnetic resonance signal. This signal can then be detected by means of an appropriate detection module.
Gradients of the magnetic induction B0 can be used in various spatial directions, so as to have different induction values between two points in space, each corresponding to an elementary volume of the body in question.
The use of magnetic induction B0 gradients therefore allows spatial localization of the signal. The step of coding the space by means of the gradients is carried out between the proton excitation and the magnetic resonance signal reception. These basic principles give rise to different methods of exploitation so as to allow the production of a selective image for a chosen element of the body observed, for example a blood vessel.
In a first method, referred to as “time of flight” method, the radio frequency waves are transmitted repeatedly and regularly, in a train of pulses. The repetition of these waves is adjusted so as to be sufficiently frequent for the proton relaxation not to have time to be entirely complete before transmission of the next wave. This saturation phenomenon means that the magnetic resonance signal is greatly reduced. It virtually makes it possible to eliminate the signals transmitted by the immobile protons, i.e. typically the protons that are part of tissues of the body in question.
On the other hand, mobile protons that penetrate the zone in question without having been subjected beforehand to a train of pulses come into resonance and create a magnetic resonance hypersignal that can be detected. The mobile protons are typically the protons contained in the water of the circulating blood.
This time of flight method therefore makes it possible to distinguish between the relaxed mobile protons and the saturated immobile protons and thus makes it possible to isolate a selective signal corresponding, for example, to a blood activity. This method can in particular be applied in the field of angiography, since it makes it possible to detect a signal originating from a blood vessel in particular.
It is, however, limited to the analysis of blood vessels that are short and have a high flow rate, since, if the opposite is true, the protons contained in the blood circulating in these vessels rapidly undergoes saturation, like the protons of the surrounding tissues.
A second method, referred to as “phase contrast” method, takes advantage of the relationship that exists between the phase of the detected magnetic resonance signal and the rate of proton displacement in the body in question, to allow detection of blood vessels within the body. However, this method has drawbacks insofar as a prior estimation of the rate of circulation in the vessels is necessary. In addition, since the phase is a quantity expressed to within 2π, an ambiguity remains regarding the effective rate deduced from a magnetic resonance signal.
These first two methods are therefore based on characteristics associated with a displacement, in particular of blood in the body. They thus find an application in the angiography field. On the other hand, they do not make it possible to detect a particular static or virtually static element of the body. They cannot therefore be used as a basis for the formation of an image for a particular organ or for a particular cell type.
A third method has made a name for itself in the last few years in the angiography field. It comprises a step consisting in injecting a contrast product into a body. In general, the contrast product used is gadolinium attached to a chelating agent such as DOTA (or tetraazacyclododecane tetraacetate) or DTPA (or diethylenetriamine pentaacetate). The chelating agent is a molecular cage that surrounds the gadolinium and makes it possible to limit its toxicity with respect to the body into which it is injected. The effect of this product is to decrease the relaxation time of the protons that are in proximity. Specifically, the contrast product contains single unpaired electrons which have a paramagnetic effect that acts on the water protons.
This increase in proton relaxation makes it possible to limit the saturation in the zone where the injected product is located. The resulting magnetic resonance signal is therefore greatly increased. Conversely, the protons that are not in immediate proximity to the gadolinium keep an unchanged relaxation time and therefore generate a lower magnetic resonance signal.
Initially after injection, the contrast product moves in the blood vessels without being absorbed by the surrounding tissues. Detection of the magnetic resonance signals therefore makes it possible to distinguish between the blood vessels and the surrounding tissues and also to form an image revealing this distinction.
However, this technique also has drawbacks. In particular, paramagnetic gadolinium, in addition to its action on proton relaxation time, creates magnetic induction microgradients that result in local distortions of the magnetic induction to which the body is subjected. The frequencies of the waves transmitted are dispersed. This effect can result in the loss of certain signals. When the magnetic resonance signals are used to form an image of a zone of the body in question, said image will therefore be difficult to interpret. This results in the spatial resolution of the images obtained by this technique being limited: this method does not allow complete suppression of the signals derived from tissues lacking contrast product.
An object of the present invention is to provide a method for acquiring magnetic resonance signals that limits the problems encountered in the above techniques.
Another object of the invention is to enable acquisition of the signals from a selected observed zone, independent of its type. For example, the observed zone may contain substantially mobile or substantially immobile protons. It may be a blood vessel or a vascularized network, but also an organ, a group of cells, or the like.